PROGRESS IN THE STUDY OF VARIATION. 561 



facts are that this striking black variety, of which the early 

 entomologists say nothino, was first met with as a rarity 

 in the North of England towards the end of the forties. 

 It was figured and named by Milliere (6) from an English 

 specimen. The variety lives on the various food plants of 

 the type. In 1865 it is reported that most of the males 

 attracted by females exposed at Manchester were of this 

 dark form. In a few years it had spread through a great 

 part of the Midlands. In 1870 the black form was almost 

 as frequent as the type at Newport, Monmouthshire, and a 

 few years later the typical form had almost entirely vanished 

 from that district. At Chester none but black specimens 

 have been met with for many years. In recent years 

 casual specimens have been taken in various English 

 localities as far south as Berkshire. On the continent the 

 appearance of the black variety was much more recent. A 

 dark specimen — though not so dark as doubledaya7Ha — was 

 recorded in Belgium in 1886 {4), and again intermediates 

 are reported from Belgium in 1894 (s)- I" ^^ P^^^ ^^" 

 years the black form has spread over a great part of Ger- 

 many, as far as Silesia and Dresden (8) (p. 316). 



Here then is an actual case of the establishment of a 

 natural variety. If only the Lancashire entomologists had 

 put by 100, or even twenty, specimens o{ betularia taken at 

 random year by year, what a splendid record it might have 

 been ! But though there is no such complete evidence as 

 this, the leading fact is clear. In a short time, some 

 fifty years, a dark form has supervened on the light one. 

 Now did this come about by the gradual shifting of the 

 average form through all stages of grey to black? It is 

 practically certain that it did not. The change came about 

 by the success of a particular dark strain or strains. Not 

 that it must be supposed that intermediates are unknown. 

 On the contrary, Mr Barrett, who kindly showed me 

 several of them, tells me that he believes that they were 

 formerly more common than they are now. Moreover, in 

 Belgium it seems clear that an intermediate strain has 

 established itself, and intermediates are also said to be 

 plentiful in the Rhenish Provinces and Westphalia, together 



